Can only speak for our 2007 Montana 5er as I have no experience with the others.
Bought new in July '07. Have been really amazed at how few problems we have had with it. And the few problems have been things not directly associated with the trailer itself. Propane regulator valve failed at about 3 yrs, microwave/convection failed after about 3 yrs, no-name surround sound/DVD system failed at about 4 yrs. Black tank flush valve failed at 6 yrs. Those are about the only problems we have had. Montana has been the leading seller for 11 years straight and I guess that speaks for itself.
I also frequent the very active Montana Owners Club (MOC) forum. And by and large most Montana owners are very happy with their purchases and buy new Montanas when they get ready to trade. Not to say they are entirely trouble free as some have reported more than what would be considered normal problems. But with MOC backing Keystone has generally resolved those problems to the buyers satisfaction. MOC holds multiple regional rallys each year and a large yearly rally in Elkhart with Montana representatives attending to hear complaints/suggestions and to give tech tip siminars.